1. Field of the Invention
The present invention relates to a method of computing an engine intake air amount for determining a basic injection amount of a fuel supplied to an engine, and a method and apparatus for executing an air fuel ratio control using the computing method.
2. Description of the Related Art
The engine intake air amount is an important data for determining a basic injection amount of the fuel. An accurate air fuel ratio control can be executed by determining the fuel injection amount by using the engine intake air amount. For example, as described in Japanese Unexamined Patent Publication No. 58-32964, the engine intake air amount is generally computed by metering an intake pipe air flow rate per a unit time by a flow meter such as a vortex flow meter placed in an upstream side of a throttle valve in the intake pipe. However, since the intake pipe has various lengths and shapes as well as an air density correction is necessary in this method, it is hard to accurately know an amount of air which is actually sucked into a cylinder, so that it is not easy to control a fuel injection valve so as to achieve a theoretical air fuel ratio.
On the other hand, there has been known a method of computing an intake air amount per one cycle on the basis of a map, by setting an engine speed and an intake pipe pressure to an input variable. Further, as described in Japanese Unexamined Patent Publication No. 57-70934, there has been known an air fuel ratio control method of feedback controlling an air fuel ratio of air-fuel mixture supplied to the engine to a predetermined air fuel ratio by detecting the air fuel ratio on the basis of an exhaust gas component of the engine in addition to the structure mentioned above. Since this method can compute a correction amount in correspondence to a condition of the engine, it is possible to quickly respond even at an engine transition time so as to easily control to a predetermined air fuel ratio. However, since no fixed correlation exists between detected values, it is necessary to map a correction amount with respect to each of the parameters so as to store in a memory. Further, since a lot of nonlinear elements exist in an engine system, it is actually very hard to determine an accurate intake air amount from the map so as to execute an accurate air fuel ratio control.